This invention relates to a distributor-type fuel injection pump for internal combustion engines, and more particularly to a distributor-type fuel injection pump having an injection timing control device which is adapted to control the internal pressure within the suction space so as to adjust the fuel injection timing characteristics.
In a conventional distributor-type fuel injection pump for internal combustion engines, the fuel pressure within the suction space (hereinafter merely called "the internal pressure") is controlled solely by a pressure regulating valve. Accordingly, the internal pressure varies linearly in proportion to a change in the rotational speed of the fuel injection pump, i.e. that in the rotational speed of the engine. As a result, an injection timing control device (a timer) provided in the pump and adapted to operate in response to a change in the internal pressure for control of the fuel injection timing can merely provide a fuel injection timing characteristic variable linearly relative to the engine rotational speed.
It is generally employed to retard the fuel injection timing in order to reduce the amount of noxious ingredients such as NO.sub.x in the exhaust gases emitted from the engine. However, in the conventional distributor-type fuel injection pump, it is not possible to retard the fuel injection timing in a particular engine speed region due to the above-mentioned linear fuel injection timing characteristic, thus failing to fully satisfy requirements imposed by the engine.
In order to satisfy the requirements by the engine, a distributor-type fuel injection pump has been proposed by Japanese Utility Model Publication No. 58-22994 or by Japanese Patent Provisional Publication No. 55-151124, wherein a member operable in response to the internal pressure such as a timer piston of the injection timing control device and the pressure regulating valve is formed therein with a fuel drain passage which is opened and closed in response to changes in the internal pressure, so as to vary the internal pressure.
However, in the proposed fuel injection pump, the fuel passage is merely opened or closed in response to the internal pressure, resulting in that the injection timing characteristic has a large hysteresis margin, i.e. a large difference in the amount of fuel injection timing advance between during increase of the engine speed and during decrease of same. Therefore, the proposed fuel injection pump is not fully suited to practical use.